Udall aired an ad this week accusing Gardner of wanting to ban birth control because he backed a measure to give legal rights to fertilized eggs. Planned Parenthood also hit Gardner on the issue in an ad Thursday.

Gardner has since disavowed the so-called personhood proposal. In a piece for the Denver Post, he said it would be safer and cheaper if the pill was available over the counter.

Gardner and Udall are competing for the votes of a small number of suburban women who often decide statewide Colorado elections.